1. Field of the Invention
The present invention relates to a constant-current control method for a constant voltage source and the like, and an electrophotographic image forming apparatus such as a printer or a copying machine using such a constant-current control method.
2. Related Background Art
First of all, an example of a conventional constant-current control method in which a constant voltage source is used and the constant voltage source is controlled by a CPU so that detected current becomes a target current value will be explained with reference to FIG. 19. In this method, since the control is effected by the CPU with simple arrangement, control including both constant voltage and constant current can be effected, and a constant voltage value and a constant current value can freely be set so that fine control can be permitted. Thus, this method is suitable for an image forming apparatus and is particularly applied to high voltage control for an electrophotographic image forming apparatus.
A matter 200 to which voltage is applied (referred to as "voltage applied matter" hereinafter) is connected to a constant voltage power source 210 and a current detect means 220 for detecting flowing current, and the power source 210 and the detect means 220 are connected to a CPU (control means) 230. When the voltage is applied from the constant voltage power source 210 to the voltage applied matter 200, a detection signal from the current detect means 220 is inputted to the CPU 230, and, on the basis of the inputted signal, the CPU 230 controls the constant voltage power source 210 so that the current becomes a predetermined current value.
The voltage applied matter 200 may, for example, be a first charge means, a developing means, a transfer means, a transfer material absorbing means or an electricity removing means of an electrophotographic image forming apparatus.
Next, the above control method will be fully described with reference to a flow chart shown in FIG. 20.
When the constant-current control is started to apply the voltage V to the voltage applied body 200 (step 1), a value of the current generated by the apply voltage is detected by the current detect means 220 and then is converted into an analogue signal of 5V which is in turn inputted to the CPU 230. The detection signal inputted to the CPU 230 is A/D-converted into 8-bit value (step 2).
A current value i obtained in this way is compared with a target current value i0 in the CPU 230 (step 3). If a difference (target current value-present current value) is positive (plus), a voltage value obtained by adding a predetermined voltage change width .DELTA.V to the present voltage value V is set (step 4), and, if the difference is zero, the present voltage value V is set as it is (step 5), and, if the difference is negative (minus), a voltage value obtained by subtracting a predetermined voltage change width .DELTA.V from the present voltage value V is set (step 6). After it is judged whether the control is finished or not (step 7), if negative, the constant voltage power source 210 applies the voltage V to the voltage applied matter 200 in accordance with the voltage value outputted from the CPU 230 (step 8). By repeating this operation, the current is converged to the target current value, thereby achieving the constant current control.
FIG. 22 schematically shows a photosensitive drum (image bearing member) 111 and a transfer device 115 of a full-color image forming apparatus using the above-mentioned conventional constant current control. Incidentally, in FIG. 22, process means for forming an image such as a charge device, an exposure device, a developing device (with yellow, cyan, magenta and black toners) and a transfer means are omitted from illustration.
Through the image forming processes such as charging, exposure and developing, a yellow toner image is formed on a surface of the photosensitive drum 111 rotated in a direction shown by the arrow R1.
The yellow toner image is transferred onto an intermediate transfer belt (intermediate transfer member) 115a of the transfer device 115. The intermediate transfer belt 115a mounted around rollers 115b, 115c and 115d is rotated in a direction shown by the arrow R5. By applying first transfer bias to a first transfer roller 115e, the yellow toner image on the photosensitive drum 111 is first-transferred at a first transfer nip T.sub.1. Similarly, a magenta toner image, a cyan toner image and a black toner image are successively formed on the photosensitive drum 111 and are successively first-transferred onto the intermediate transfer belt 115a. As a result, four color toner images are superimposed on the intermediate transfer belt 115a. The four color toner images are secondary-transferred onto a transfer material P such as a paper sheet at a secondary transfer nip T.sub.2 by applying secondary transfer bias to a secondary transfer roller 115f from a constant voltage power source 210 by using the above-mentioned constant current control method.
After the secondary transferring, the transfer material P is sent to a fixing device (not shown), where the toner images are fixed to the transfer material. However, in the above-mentioned constant current control method, during the control, if a load is varied abruptly, there arises a problem that it takes a long time to converge the current to the target current value.
As shown in a graph in FIG. 21, although the converging time t depends upon the voltage change width .DELTA.V, if the voltage change width .DELTA.V is great, merely when the current is slightly deviated from the target current value, the voltage is greatly changed not to converge the current. Therefore, the voltage change width .DELTA.V must be small, with the result that, if the load is varied abruptly, it takes a long time to converge the current to the target current value.
Further, if noise is generated in the current detection signal inputted from the current detect means 220 to the CPU 230 to vibrate the current detection signal, there arises a problem that, even after the control, the voltage is vibrated. To solve this problem, the generation of noise should be disposed of or noise should not be carried on a signal in a communication path, but it is very expensive to eliminate noise completely.
Further, in the above-mentioned conventional constant current control method, apart from the above-mentioned load variation, there arises a problem that it takes a long time to converge the current from the initiation of the control. The reason is that, since the control is started at the same time when the first voltage is applied, the control is effected before the voltage is risen up, with the result that overshoot of current occurs to delay the convergence of the current.
Further, in the above-mentioned image forming apparatus, there arises a problem that the transferring property of the secondary transferring is changed in accordance with the condition of the transfer material P (for example, a pass width of the transfer material (a length of the transfer material in a direction perpendicular to the conveying direction)), or a process speed, or a mono-color image or a full color image, or a one-face mode or a both-face mode, or resistance of the transfer material. This is caused by the fact that, in dependence upon the condition of the transfer material, a ratio between current flowing through the sheet pass portion (portion where the transfer material exists at the transfer nip T.sub.2) and the sheet non-pass portion (portion where the transfer material does not exist at the transfer nip T.sub.2) is changed.
Regarding the variation of the pass width, in order to stabilize the transfer property, although the current flowing through the sheet pass portion must be constant, in the above-mentioned constant voltage power source 210, since the control is effected so that the sum of the current flowing through the sheet pass portion and the current flowing through the sheet non-pass portion becomes constant, if the pass width of the sheet is changed, the transfer property is changed.
Further, when a distance between the secondary transfer roller 115f and the fixing device is selected to become smaller than a maximum size of the transfer material (length of a maximum transfer material in the conveying direction) to make the image forming apparatus compact, the longer transfer material may be subjected to the influence from the secondary transfer roller 115f and the fixing device simultaneously. In such a case, if a fixing speed is reduced to ensure the good fixing ability, a transfer speed must be reduced accordingly. If the transfer speed is changed in this way, in the constant current control effected by using the constant voltage power source 210, the transfer property will be varied. The reason is that, in the constant voltage power source 210, since the control is effected with the same current value even when the transfer speed is changed, an amount of shifted charges per unit area is changed.
Furthermore, the transfer property is changed in accordance with the mono-color mode or the full-color mode. The reason is that, in the mono-color mode, the intermediate transfer belt 115a is rotated only by one revolution to form the image on the transfer material P. Whereas, in the full-color mode, since the intermediate transfer belt 115a is rotated at least by four revolutions, the charge amount of toner (before the secondary transferring) on the intermediate transfer belt 115a differs between the mono-color mode and the full-color mode, with the result that the optimum current value of the secondary transferring is changed.
Further, the transfer property is changed in accordance with the one-face mode (in which an image is formed on a single surface of the transfer material) or the both-face mode (in which images are formed on both surfaces of the transfer material). The reason is that, in the both-face mode, when the image is formed on a second surface of the transfer material, after the image was formed on a first surface of the transfer material, since the transfer material is once passed through the fixing device to be heated thereby, moisture is vapored to change the resistance of the transfer material.